The present invention relates generally to reinforced composites, and more particularly, to a two-speed insertion process for Z-pinning/joining uncured composite laminates to each other.
The use of composites as primary structures in aerospace applications is becoming increasingly widespread in the aerospace industry. Traditional composite materials are made up of a resin matrix material and a quantity of two-dimensional fibers, continuous in the X-Y axis direction, but laminated in layers to produce a material thickness. Composite material construction, wherein a fiber material such as a glass fiber, carbon fiber, or aramid fiber is combined with a matrix material, such as thermoplastic or thermoset resins, is an example of a traditional two-dimensional structure.
Many structural composites, such as structural composite air frames, usually include multiple stiffeners. The stiffeners supply rigidity and stiffness that is required under certain flight load conditions. One typical stiffener is referred to as a hat stiffener. Hat stiffeners, named for their shape, are typically applied to aerospace structural composite components via their skin.
Historically, composite hat stiffeners were attached to composite skins with conventional mechanical fasteners. In another attachment process sometimes employed, the hat stiffeners were co-cured to the skin of the structural composite material concurrently with the curing with the structural composite material itself. However, in both this process and that wherein the hat stiffeners were mechanically bolted and/or adhesively bonded to the skin, the failure mode typically occurred at the inner hat stiffener to skin surface.
In order to resolve the occurrences of failure using the aforementioned attachment processes, Z-pinning is now frequently used in the aerospace industry to facilitate the attachment of one or more stiffeners to a composite skin. In this regard, with the development of Z-pins, methods for supporting the Z-pins in a carrier, and methods for inserting the Z-pins into uncured composite materials, the hat stiffeners and skin are able to be joined to each other prior to being cured. Joining composite parts together with Z-pins offers several advantages over conventional mechanical fasteners, such as lighter weight, more even distribution of the load, lowers costs, and co-curing of the two parts. In one currently employed Z-pinning process used in conjunction with hat stiffeners, a Z-pin carrier pre-form is disposed on that surface of the hat stiffener which is to be secured to the skin of the underlying structural composite material or laminate. The pre-form typically comprises contiguous layers of low and high density foam having a multiplicity of Z-pins embedded therein. The Z-pins are forced from the carrier pre-form through the hat stiffener and into the underlying laminate using a device such as a hydraulic press or an ultrasonic device (e.g., an ultrasonically excited horn) which uses high frequency energy to vibrate the Z-pins within the carrier pre-form to force them through the stiffener and into the underlying laminate.
For purposes of achieving greater efficiencies and economies in the Z-pinning process, it is highly desirable to facilitate the insertion of the Z-pins automatically through the use of a robot. However, attempts at automating the Z-pinning process have proven challenging due to the need for special techniques to accommodate the many variables involved with the Z-pinning process. More particularly, the key variables for automated insertion are insertion speed, insertion force, material age, material thickness, amount of laminate hot debulking, amplitude of the excitation of the horn (when an ultrasonic horn is used), the load bearing capability of the Z-pins, and insertion time. Inserting the Z-pins too fast results in excessive force being applied to the pins, thereby crushing them, or causing them not to penetrate completely through the parts being joined. Inserting the Z-pins too slowly takes excessive time thereby not achieving a reasonable return on investment, or causes the pre-form to overheat which creates a potential for a fire hazard. When an ultrasonic horn is used, increasing the amplitude of the horn oscillation allows for faster insertion, but increases the risk for transferring too much energy into the pre-form causing over-insertion of the Z-pins and melting of the pre-form. Moreover, one set of conditions may be fine for a new material and a thin total thickness, but not optimal for an aged material (e.g., a thirty day age material) that is of a maximum thickness. Indeed, since many of the aforementioned variables interact in a non-linear fashion, it is extremely difficult to predict insertion success with any given combination. As an additional restriction, the insertion time is required to be fast enough to generate a good return on investment for the process. Thus, for automated Z-pin insertion in a production environment, there exists a need in the art for a single set of insertion parameters (universal parameters) which accommodate all the variations likely to be encountered within the specifications.
The need for a single set of insertion parameters for use in an automated Z-pin insertion process is addressed by Applicant's co-pending U.S. application Ser. No. 11/158,400 entitled AUTOMATED Z-PIN INSERTION TECHNIQUE USING UNIVERSAL INSERTION PARAMETERS filed Jun. 22, 2005, the disclosure of which is incorporated herein by reference. However, with currently known manual insertion processes, follow-up manual inspection steps are still typically required to verify that the Z-pins were inserted to the full, proper depth. More particularly, in manual inspection processes as currently practiced, the height of the Z-pin carrier pre-form is measured upon the completion of the insertion process through the use of a simple gage to determine if the Z-pins were pressed far enough into the composite laminate to reach full depth. The definition of full depth in this case means that the Z-pins have fully penetrated the composite laminate and reached the bond tool surface. Following cure, an additional visual check is performed to verify that the Z-pins did in fact penetrate the composite laminate ply closest to the tool surface. It is contemplated that a similar manual inspection process may be implemented in relation to the automated Z-pin insertion process/system described in Ser. No. 11/158,400 to verify that the Z-pins were inserted to full depth.
When implementing an automated Z-pin insertion process through the use of an ultrasonic horn and a Z-pin carrier pre-form, both full grid and partial grid Z-pin insertions usually occur. A full grid insertion occurs when the footprint of the entire anvil of the ultrasonic horn lies within or rests upon the exposed top surface of the carrier pre-form. A partial grid insertion occurs when a portion of the anvil extends beyond one of the sides or peripheral edge segments of the carrier pre-form. When inserting Z-pins into a composite structure through the implementation of an automated Z-pinning process, the corresponding robotic system is not able to detect the difference between a full grid and a partial grid insertion using only the load cell force feedback described with particularity in Ser. No. 11/158,400. Neither can the automated Z-pinning system detect when inferior Z-pins are present in the carrier pre-form. Inferior Z-pins are unable to carry the normal insertion forces exerted by the robotic system and thus often crumble instead of fully inserting into the underlying composite structure.
During normal full grid insertions, the automated Z-pinning systems described in the aforementioned related applications of Applicant work extremely well. However, at the beginning and end of each Z-pin carrier pre-form, a partial grid insertion usually occurs. The effect of inserting this partial grid is to cause over insertions, which typically causes one or more of the following conditions: (1) the Z-pins on top of the carrier pre-form are crushed, thus causing a blackening of the surface of the underlying substrate and creating a cosmetic problem; (2) the time to complete the insertion is increased, often to a maximum value of six seconds, thus wasting extra time for no purpose; (3) the carrier pre-form is heated up beyond normal, causing the low density foam (polystyrene) included therein to melt; and (4) the maximum insertion depth for the Z-pins is often reached, again wasting valuable time. Occasionally, the quality of the Z-pins themselves is questionable as they sometimes are unable to carry the insertion force even on a full grid insertion, thus causing the Z-pins to crumble and not fully insert. This tends to be a more serious problem than over insertion as it could result in the parts not being pinned with enough integrity to maintain structural strength in the joint. The present invention addresses these particular problems by providing a method of detecting the above-described undesirable conditions in a manner which will be described in more detail below.